Just over a week ago, representatives of 196 parties around the world filed into a final plenary session at the Paris climate talks. After alast-minute hiccup, the gavel came down, and theParis agreement on climate change was finally adopted.

The agreement requires countries to update and enhance their targets to cut greenhouse gases every five years, but the targets themselves are not legally-binding. In the absence of enforceable commitments to reduce emissions, concerned citizens may look to the courts for an alternative way of driving emissions cuts by countries or companies.

We take a look at how litigation – the process of resolving disputes in a court of law – could have an important role to play.

Two aspects of causation

Consider this example: how could climate litigation be used to prove liability for a company, say a fossil fuel giant, towards a claimant for climate change damage?

A case like this would fall under tort law, which is used to determine liability when actions by one person or company have caused some specific harm to another. A claimant in a climate change case will have to convince the court of two things.

First, the court will have to accept that greenhouse gas emissions from human activity are the principal driver of climate change. A few months ago, we sawjudges in the Netherlands do exactly that. A case was brought against the Dutch government by theUrgenda Foundation and 900 Dutch citizens, who argued that the government was in the wrong for not taking sufficient action to prevent foreseeable harm from climate change.

The second aspect of causation is more complicated to prove. The claimant needs to convince the court that human-caused emissions are behind the climate event alleged to have caused harm.

Such a case would also need a defendant. In our example, let’s say it’s a coal-burning utility company.

If the court agrees that its emissions contribute to climate change, under tort law, the crucial question is then whether the utility company has a duty of care to the claimant. In the case of climate change, a court could rule that the utility company contributed to the harm in a way that can potentially be quantified with reasonable certainty.

However, this raises a problem: the utility company isn’t the only contributor to climate change.

To have a clear example of how a court might tackle this, we can look at how English courts have used epidemiological evidence in health cases.Epidemiology is the study of health patterns in the general population; it is used to investigate the causes and prevention of diseases.

Let’s look at some examples. In these cases there is a clear harm, but difficulty in showing that the harm was caused by a specific defendant.

InXYZ versus Schering Health Care, the claimants argued that contraceptives they were prescribed causeddeep vein thrombosis. The trial court considered the evidence and came to a numerical conclusion on risk. It decided that the contraceptives increased the risk by 1.7 times. The court ruled that this was not enough for the claimants to win, but an increase in risk of at least two times would have been enough.

A more complicated case involves asbestos, which causes a form of lung and abdomen cancer calledmesothelioma. InFairchild versus Glenhaven Funeral Services Ltd, the House of Lords carved out an exception to the “risk-doubling” rule. Mesothelioma can be caused by just a single fibre of asbestos, so a claimant may have worked for several companies, but will never be able to prove where he or she inhaled the deadly fibre. The Court, therefore, decided to hold the employers jointly liable and spread the damages between them.

That’s not to say that exceptions will often be made. In the most recent epidemiology case seen by theSupreme Court – where former soldiers exposed to fallout from nuclear testing sued the Ministry of Defence – the Court decided not to extend the Fairchild exception to the risk-doubling rule.

Attribution

As success in a tort case revolves around being able to prove how actions led to harm, climate cases will rest heavily on scientists being able to attribute the strength or likelihood of an extreme weather event to climate change.

For example, arecent study found that a summer as hot as the European heatwave of 2003, which killedat least 70,000 people, is now ten times more likely because of human-caused greenhouse gas emissions.

So the risk of heatwaves has far more than doubled, an increase that the court was comfortable with in the contraceptives case. However, in that case, there was a single defendant responsible for the harm. Even though the total probability for a heatwave has much more than doubled with climate change, a large group of actors are responsible. None of them individually has come close to doubling the risk.

The multiple contributors to the increase in greenhouse gas and their resulting damage makes the heatwave case more comparable to the mesothelioma case, where the claimant has multiple employers who were all liable.

Let’s say that you could show that the utility company is responsible for 3.2% of total human-caused climate change. Should judges simply apportion 3.2% of the damages for, perhaps, a heatwave death or storm damage, to them?

This solution is appealingly simple, but the utility company could argue that it would be unfair to make it pay damages without a contribution from all other emitters – even individual car drivers. This would quickly make the case unworkable.

Remedies for the future

Even if the utility company lost that argument, it is not easy for the courts to determine the correct outcome, or “remedy.”

At arecent event, Supreme Court judges and scholars from around the world discussed the issue. They agreed that courts may be reluctant to fine key carbon emitters for existing climate damages because the liability could become extremely large. Total damages from climate change could be somewhere between2% and 20% of GDP per annum, so even a giant company with deep pockets would feel the pinch. Jurists at the conference felt, however, that a reasonable approach would be to ask for a remedy that is future-facing rather than backward-looking.

Our hypothetical case could look as follows: assume a court agreed that the damage was caused by human-caused climate change, and the defending utility company was – among other parties – liable for the harm. Rather than ask for monetary compensation for damage done, the claimant asks that the company significantly restrict their emissions – driving future benefits for the climate.

The interesting question is then whether a court would grant this remedy.

But such a case is still only hypothetical. For the time being, we need scientists to keep developing robust climate attribution science and lawyers to keep devising innovative ways to hold major emitters to account. This, maybe more than Paris Agreement, could be the best way to protect people and the planet.